The early detection of cancer using medical imaging equipment requires the ability to detect small lesions or to delineate the boundaries of lesions that have properties close to those of the surrounding normal tissue. The measure of the smallest object visible with a given contrast is called the resolution of the imaging system. Contrast resolution and other performance tests of a medical imaging system are performed with objects called phantoms. A phantom with low contrast, like that of tissues, is required for the evaluation of the contrast resolution of the system. Such phantoms are commercially available for use with X-ray computed tomography and ultrasound imaging systems but are not generally available for nuclear magnetic resonance (NMR) imaging systems. NMR has particular advantages in investigations of the spinal cord and knee where bone impedes X-rays and ultrasound, and for evaluation of cardiac performance without using contrast agents as are needed in X-ray angiography.
Hydrogen magnetic resonance imaging is generally a more complicated imaging procedure than X-ray or ultrasound since it does not measure just one dominant property, such as electron density in the case of X-ray computed tomography, but is affected by the hydrogen atom density, flow, and two relaxation phenomena. The contrast, or differences in image brightness, in an NMR image is primarily due to differences in the relaxation times of tissues. It has been found that there are relaxation time differences between normal tissue and certain tumors, which makes NMR imaging potentially very valuable in early detection of such tumors.
A satisfactory NMR phantom must satisfy several, sometimes conflicting requirements. First, the material of which the phantom is made should mimic the hydrogen density and relaxation times of several types of tissues. Second, the relaxation times of the material should not change over time, such as over several months or years, so that the phantom can be used in tests of imager reproducability. Third, if the phantom includes inclusions of materials within the surrounding matrix which have different NMR characteristics than the surrounding matrix, these inclusions must be stable over time in both shape and in NMR relaxation times, T.sub.1 and T.sub.2.
Soft tissues exhibit T.sub.1 's ranging from about 200 milliseconds (ms) to 1200 ms and T.sub.2 's from about 40 ms to 200 ms. Typical values for the ratio T.sub.1 /T.sub.2 lie between 4 and 10 for soft tissues. For a given soft tissue parenchyma, T.sub.1 in particular can exhibit a significant dependence on frequency as well as temperature.
Materials which have been proposed for use in phantoms to mimic soft tissues with respect to one or more NMR properties include aqueous solutions of paramagnetic salts and water based gels of various forms. Such gels may also contain additives such as a paramagnetic salt for control of T.sub.1.
Aqueous solutions of paramagnetic salts can be used in phantoms to produce a desired value of either T.sub.1 or T.sub.2. The ratio of T.sub.1 /T.sub.2 in the salt solutions is almost always less than 2, however, rendering such solutions inadequate for the close mimicking of soft tissue, with the possible exception of body fluids.
Phantom materials composed of water based agar gels doped with MnCl.sub.2 to control T.sub.1 have been reported. R. Mathur-DeVre, et. al., "The Use of Agar as a Basic Reference for Calibrating Relaxation Times and Imaging Parameters, " Magn. Reson. Med., Vol. 2, 1985, p. 176. Agar gels doped with CuSO.sub.4 have also been reported. M. D. Mitchell, et al., "Agarose as a Tissue-Equivalent Phantom Material for NMR Imaging," Magn. Reson. Imag., Vol. 4, 1986, p. 263. To produce sufficiently low values of T.sub.1 /T.sub.2 for mimicking soft tissues, a rather low dry weight concentration of agar must be used (1% to 2%). When employing agar gels of these low concentrations in sealed glass containers, slow shrinkage of about 2% in volume over a period of months has been observed, with fluid being extruded at the boundaries, making such materials unsuitable for forming complex phantoms such as contrast resolution phantoms or anthropomorphic phantoms.
A material in which very high dry weight concentrations of agar and animal hide gels were employed to control (i.e., lower) T.sub.1 without use of paramagnetic salts has been reported by W. T. Dixon, "Simple Proton Spectroscopic Imaging," Radiology, Vol. 153, 1984, p. 189. Production of these very high gel concentrations apparently requires considerable time, effort and care. Failure to produce stable complex phantoms was also reported.
A polyvinyl alcohol gel is described by I. Mano, et al., "New Polyvinyl Alcohol Gel Material for MRI Phantoms," Magn. Reson. Med. Vol. 2, 1986, p. 921. This material appears to lack the long term stability desired in phantoms; the relaxation times reportedly decreased 4% to 12% in six months. Another disadvantage exhibited was extrusion of fluid at boundaries of the material. A polyacrylamide gel material proposed as a tissue mimicking material is described in F. DeLuca, et al., "Biological Tissue Simulation and Standard Testing Material for MRI," Magn. Reson. Med., Vol. 4, 1978, p. 189.
A phantom material consisting of mixtures of agar gel and animal hide gel in which CuSO.sub.4 was used to lower T.sub.1 has also been reported. Unfortunately, a long-term instability manifested itself in that a steady, very slow rise in T.sub.1 was observed over a period of months. This instability precludes the use of this material in MRI phantoms. The rise in T.sub.1 was perhaps due to the slow formation of metal-organic complexes, removing the Cu.sup.++ paramagnetic ions. J. C. Blechinger, et al., "NMR Properties for Tissue-Like Gel Mixtures for Use as Reference Standards or in Phantoms," Med. Phys., Vol. 12, 1985, p. 516 (Abstract).
E. L. Madsen, et al., "Prospective Tissue-Mimicking Materials for Use in NMR Imaging Phantoms," Magn. Reson. Imag., Vol. 1, 1982, p. 135, reported water-based animal hide gels which depended upon the concentration of glycerol for control of T.sub.1 and on the concentration of graphite powder for control of T.sub.2. Unfortunately, the instrument used in the work reported on in that article employed what has become known as the simple Hahn Spin-Echo Pulse Sequence for Measuring T.sub.2. Later measurements, made with an instrument using the Carr-Purcell-Meiboon-Gill (CPMG) pulse sequence, expose a strong dependence of the apparent T.sub.2 on 2, the time between 180.degree. pulses. No well defined T.sub.2 could be established for the materials using the CPMG pulse sequence. It is likely that the microscopic diamagnetic graphite particles caused inhomogeneities in the magnetic induction, B.sub.0, to such an extend that even the CPMG pulse sequence was unable to eliminate their effect.